1. Field of the Invention
The present invention relates to a powertrain and a method of controlling a powertrain, and in particular, to a technique of controlling a powertrain having an engine and a rotating electric machine as motive power sources.
2. Description of the Background Art
Conventionally, a hybrid vehicle having an internal combustion engine and a rotating electric machine as motive power sources is known. In such a hybrid vehicle, the internal combustion engine and the rotating electric machine are selectively used in accordance with a traveling state of the vehicle. For example, the internal combustion engine is mainly used for traveling at a high speed, and the rotating electric machine is mainly used for traveling at an intermediate or low speed. One such hybrid vehicle is provided with a differential mechanism that functions as a continuously variable transmission by means of a rotating electric machine.
Japanese Patent Laying-Open No. 2005-337491 discloses a control apparatus for a vehicular drive apparatus. The control apparatus includes: a continuous shift portion that has a differential mechanism constituted of a first element coupled to an engine, a second element coupled to a first motor (rotating electric machine), and a third element coupled to a second motor, and that functions as an electric continuously variable transmission; and a shift portion (shift mechanism) provided between the continuous shift portion and wheels. The control apparatus of Japanese Patent Laying-Open No. 2005-337491 includes a continuous shift control portion that executes, when the shift portion is shifted, shift of the continuous shift portion synchronizing with the shift of the shift portion so that the gear ratio implemented by the continuous shift portion and the shift portion is continuous.
According to the control apparatus disclosed in the publication, the gear ratio implemented by the continuous shift portion and the shift portion, i.e., a synthesis gear ratio implemented based on the gear ratio of the continuous shift portion and that of the shift portion is continuously changed. Thus, the engine speed (revolution speed) is changed continuously before and after the shift of the shift portion, whereby a shift shock is reduced.
Meanwhile, in a hybrid vehicle having a powertrain provided with two rotating electric machines such as the vehicular drive apparatus disclosed in Japanese Patent Laying-Open No. 2005-337491, for example when the shift lever is in P (parking) position, one of the rotating electric machines may be driven by the engine to generate power. In such a situation, in the vehicle provided with a stepwise transmission between a continuous shift portion, wherein an engine and two rotating electric machines are coupled via a differential mechanism, and wheels, such as the one disclosed in Japanese Patent Laying-Open No. 2005-337491, when the stepwise transmission in P (parking) range or the like is shifted to the neutral state, in order for the driving force of the engine to efficiently be transmitted to the rotating electric machine employed as a generator, it is desirable that a rotary element coupled to the rotating electric machine that does not generate power is stopped. Accordingly, when power is generated by one of the rotating electric machines, control is provided over the other rotating electric machine for allowing the rotary element coupled to the other rotating electric machine to stop. However, when control over the rotating electric machine for allowing the rotary element to stop is provided, if the vehicle is to be started, torque of the rotating electric machine must once be “0” so that transition to control for allowing the rotating electric machine to actuate as the drive source of the vehicle is realized. Here, since the rotary element having been stopped becomes freely rotatable, the load to the engine is abruptly reduced and the engine speed may abruptly be increased.